A powder coating system sprays an electrostatically charged airborne powder within an enclosure or booth containing the part or article to be coated. The electrostatic potential between the powder and the article causes the powder to be attracted to and move into contact with the surface of the article. The deposited powder is then heated so that it flows and hardens on the surface on which it has been deposited.
The present invention relates to two areas of powder spray control. First is the selection and control of certain spray parameters, for example, the powder flow air pressure, the atomizing air pressure and pattern air pressure, if required. In addition, with corona type spray guns, an electrostatic voltage is selected and supplied by an internal power supply. The second area of powder spray control is gun triggering, that is, when the spray gun is turned ON and OFF, in relation to parts traveling through the spray booth. In the most basic systems, the air pressures and electrostatic voltage are controlled by manually setting respective pressure regulators and a power supply, and the gun triggering is also manually controlled.
Some systems have been developed that automate the gun triggering. For example, the "SMART SPRAY.RTM." gun controller which is manufactured and sold by Nordson Corporation of Amherst, Ohio, the assignee of the present invention, uses a microprocessor based gun controller in combination with manually set pressure regulators to automatically control the spray gun triggering. The gun controller operates with photodetectors in the spray booth to provide gun triggering in different spray booth zones. Either a conveyor feedback transducer or control timer is used with the photodetectors to detect the presence of a part as well as its front and rear edges as it travels through the booth, and the gun controller triggers the gun ON and OFF in response to the photodetectors sensing part presence. However, the spray parameters remain constant unless they are manually changed by the operator.
In other systems, a programmable logic controller ("PLC") is used as a centralized powder spray system control in association with photodetectors and a conveyor feedback transducer. The photodetectors and a feedback transducer from the conveyor sense the presence and identity of different parts, respectively, to be coated, as well as line gaps between successive parts on the conveyor. The PLC can be operatively connected to voltage to pressure transducers for selecting the desired powder air flow, atomizing air and pattern air pressures. The centralized PLC turns selected spray guns ON or OFF as a function of the part identified and line gaps between parts.
While the above systems have performed satisfactorily, they utilize a centralized controller or PLC which singularly controls the triggering of each of the spray guns, and further, singularly controls each of pressure regulators and each of the power supplies for each of the guns. This centralized system control configuration has a disadvantage of requiring extensive wiring within the painting facility much of which must be done upon installation at the user's site. Moreover, a PLC is not adept at performing complex arithmetic operations and handling more complex data structures. PLC's have the further disadvantage of only providing a limited amount of process status information to the operator or other analytical devices. Furthermore, the use of a PLC as the centralized control system has a further disadvantage in that it is difficult and expensive to change the electrical configuration of the control system. Also, there is no redundancy in a centralized PLC control system and any electrical failure within the PLC will terminate the operation of the entire coating system.
Still further, because the single centralized PLC must serially process data for each of the powder dispensers, there is a further disadvantage in that the processing bandwith, that is, the real time window in which the PLC can dedicate to processing data for a particular powder spray gun is relatively small. Therefore, more comprehensive control of the powder spray cycle is very difficult. For example, with a centralized control, a gun purge cycle to clean the dispensing hose and spray gun is not programmable. When the spray gun is triggered ON, powder is pumped from the powder source, through a dispensing hose up to thirty feet long and then through the spray gun. When the spray gun is triggered OFF, the fluidizing air pressure in the dispensing hose is terminated; and therefore, the powder in the dispensing hose separates from its transport air and often settles and collects in lumps or clumps in the hose. When the spray gun is again triggered ON, the powder lumps are sprayed in an uneven manner. With the prior control systems, a gun purge cycle is manually controlled by the operator when it is required.
Finally, as the system size, in terms of the number of powder dispensers and spray guns increases, the added complexities of using a single centralized PLC cause its costs to increase substantially.